Is this gone in 4.x or is there a way to turn it on?

There is a block called "similar items", when using the block manager.

It works pretty well, but will show only similar items according to the tags. 

If needing it based on title instead:

Actually just finished an update making it more globally available.

that is what I was looking for, thank you.

Does it have the possibility to show a thumbnail image if attached to a post / page / article or download?

that is what I was looking for, thank you.

Does it have the possibility to show a thumbnail image if attached to a post / page / article or download?

You could do that by adjusting the widget template for your theme and some logic, but that's a bit too specific to the content type in the result set I would think. I'll see what I can do in a future update.

Great, by being able to manipulate the template it will work for me.

btw, this is my template code:

{{foreach $similar as $item}}
		<div class='cCmsCategoryFeaturedEntry'>	
	
	

	    <h3>
           <a href="{$item->url()}" title='{lang="view_this" sprintf="$item->mapped('title')"}'>
               			{{if $item->record_image}}
							<div class='ipsDataItem_icon ipsPos_top'>	
							   {template="thumbImage" group="global" app="core" params="$item->record_image, $item->name"}
               				</div>
    		   			
    				    {{elseif $item->primary_screenshot}}
        			    	<div class='ipsDataItem_icon ipsPos_top'>
        				       {template="thumbImage" group="global" app="core" params="$item->primary_screenshot, $item->name"}
               				</div>
            			   
     				  
					  

                    	{{endif}}
                     
              		
              		{wordbreak="$item->mapped('title')"}
            </a>
          </h3>

	              
	
		</div>	
	{{endforeach}}

 

 Any content type could end up in that result set though, so it's not quite that simple.

I understand, but that was what I was looking for: e.g. having an article / forum post and showing similar content from downloads

I see. additional filters had been on my mind, such will indeed be in the next update, was trying to get this one out a bit more quickly due to that error noted.

Great, I am looking forward to it :-)